Disk Utility can do it. Just pop the disk in, the go to File -> New -> Disk Image from <labelofdisk>. Select CD/DVD Master as the type and No Encryption and a file with a .cdr extension appears. This is your disk image (they don't call it ISO because DVDs don't use ISO9660 file system).

-r keeps lowercase to lowercase.
-o bsd40b.iso is the end result file that is bootable and burnable.
-b 4.0/i386/cdrom40.fs is the file that gets boot when booting the cd.
-c boot_catalog adds a boot catalog and is required for a bootable cd.
-l allows for 31 character file names.
-v /Users/dreely/Parallels/OpenBSD/OpenBSD is where my content is.